The helmet (we also refer to it as a "brain protector" is to protect your head. Having cracked open a helmet on a railroad track, I appreciate how valuable the are.
The Goggles are to protect your eyes. When you are moving fast, it's impossible to react quick enough to avoid bugs, dust, and debris from getting in your eyes. And the first time a dragonfly flies into your goggles, you'll be glad you had them on ;-)